1. Field of the Invention
This invention pertains to a method and apparatus for printing characters with a high write duty cycle in an electrophotographic printer using a low power CW light source.
2. Description of the Prior Art
Electro-optic printers may employ rating multi-faceted prism scanners, galvanometer light beam deflectors or acousto-optic light beam deflectors to access the characters to be printed. The speed with which these light beam deflectors can access a character determines the printing rate of the over-all system. Acousto-optic deflectors which can randomly position a laser beam to access 2 to 5 characters in 5 microseconds or less, are commercially available but are relatively expensive. Galvanometer deflectors are available that cover a wide range of random access position and speeds; however, as the accessing speed increases, the number of Raleigh resolvable beam positions decrease and tradeoffs between accessing speed and character quality are required. Typical of these are the Honeywell models M25K and M1650 which can access approximately 20 and 1.0 characters, respectively, in one millisecond. Although rotating scanners do not provide a random access capability, a 16 faceted prism with a 45.degree. deflection angle rotating at 3600 rpm can access approximately 100 characters and provide access to each in approximately one millisecond.
A typical line of computer printout contains positions for 132 characters. Consequently, by using a high speed galvanometer deflector such as the Honeywell M25K, approximately 10,000 lines per minute may be printed. However, allowing 50% of the available time for writing information onto a photoconductive drum, and the remaining 50% for positioning the deflector, the printing speed is limited to approximately 5,000 lines per minute. The same deflector, when utilized in a scanning mode by applying a sawtooth voltage at 1/10th the resonance frequency, can access each line of 132 characters in 0.5 m sec. This results in a sequential accessing rate in excess of 200,000 characters per second and approximately 90,000 lines per minute.
The quoted rates for sequential and random accessing of character positions assume that the scanning motion must stop during random accessing and that the scanning motion is continuous during sequential accessing. Since characters can be printed on a line in a sequential fashion the higher print rate can be made practical only if the writing can take place in the presence of a continuous deflector motion. This has been accomplished in the prior art by employing a reduced laser write duty cycle to print an instantaneous series of snapshots for each sequential character. Such an approach provides unblurred characters at high print speeds but at a sacrifice in laser usage efficiency and concomittantly, an increase in cost.
The principles to be herein described enabled high printing rates to be achieved with an inexpensive light beam deflector and a CW light source.